System for real-time control of semiconductor wafer polishing including optical monitoring

ABSTRACT

A system for polishing a semiconductor wafer, the system comprising a wafer polishing assembly for polishing a face of a semiconductor wafer at a polishing rate and a polishing uniformity, the wafer polishing assembly including a platen subassembly defining a polishing area, a slurry supply system delivering a slurry to the polishing area, and a polishing head selectively supporting a semiconductor wafer and holding a face of the semiconductor wafer in contact with the platen subassembly; and an optical measurement system measuring film thickness at multiple different locations on the wafer face while the wafer is under a liquid, wherein drying of the wafer is avoided while the measurements are taken.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of U.S. patent application Ser. No.08/112,759, U.S. Pat. No. 5,486,129 filed Aug. 25, 1993, titled "Systemand Method for Real-Time Control of Semiconductor Wafer Polishing, and aPolishing Head", invented by Gurtej S. Sandhu and Trung T. Doan,assigned to Micron Technology, Inc., and incorporated herein byreference.

TECHNICAL FIELD

This invention relates to systems for polishing semiconductor wafers.

BACKGROUND OF THE INVENTION

In the fabrication of integrated circuits, numerous integrated circuitsare typically constructed simultaneously on a single semiconductorwafer. The wafer is then later subjected to a singulation process inwhich individual integrated circuits are singulated from the wafer. Atcertain stages of fabrication, it is often necessary to polish a surfaceof the semiconductor wafer. In general, a semiconductor wafer can bepolished to remove high topography, surface defects such as crystallattice damage, scratches, roughness, or embedded particles of dirt ordust. This polishing process is often referred to as mechanicalplanarization (MP) and is utilized to improve the quality andreliability of semiconductor devices. This process is usually performedduring the formation of various devices and integrated circuits on thewafer.

The polishing process may also involve the introduction of a chemicalslurry to facilitate higher removal rates and selectivity between filmsof the semiconductor surface. This polishing process is often referredto as chemical mechanical planarization (CMP).

In general, the polishing process involves holding and rotating a thinflat wafer of semiconductor material against a polishing surface undercontrolled pressure and temperature. One such apparatus for polishingthin flat semiconductor wafers is discussed in our U.S. Pat. No.5,081,796. Other apparatuses are described in U.S. Pat. Nos. 4,193,226and 4,811,522 to Gill, Jr. and U.S. Pat. No. 3,841,031 to Walsh.

One problem encountered in polishing processes is the non-uniformremoval of the semiconductor surface. Removal rate is directlyproportional to downward pressure on the wafer, rotational speeds of theplaten and wafer, slurry particle density and size, slurry composition,and the effective area of contact between the polishing pad and thewafer surface. Removal caused by the polishing platen is related to theradial position on the platen. The removal rate is increased as thesemiconductor wafer is moved radially outward relative to the polishingplaten due to higher platen rotational velocity. Additionally, removalrates tend to be higher at wafer edge than at wafer center because thewafer edge is rotating at a higher speed than the wafer center.

Another problem in conventional polishing processes is the difficulty inremoving non-uniform films or layers which have been applied to thesemiconductor wafer. During the fabrication of integrated circuits, aparticular layer or film may have been deposited or grown in a desireduneven manner resulting in a non-uniform surface which is subsequentlysubjected to polishing processes. The thicknesses of such layers orfilms can be very small (on the order of 0.5 to 5.0 microns), therebyallowing little tolerance for non-uniform removal. A similar problemarises when attempting to polish warped surfaces on the semiconductorwafer. Warpage can occur as wafers are subjected to various thermalcycles during the fabrication of integrated circuits. As a result ofthis warpage, the semiconductor surface has high and low areas, wherebythe high areas will be polished to a greater extent than the low areas.

As a result of these polishing problems, individual regions of the samesemiconductor wafer can experience different polishing rates. As anexample, one region may be polished at a much higher rate than that ofother regions, causing removal of too much material in the high rateregion or removal of too little material in the lower rate regions.

A compounding problem associated with polishing semiconductor wafers isthe inability to monitor polishing conditions in a effort to detect andcorrect the above inherent polishing problems as they occur. It iscommon to conduct numerous pre-polishing measurements of the waferbefore commencement of the polishing process, and then conduct numeroussimilar post-polishing measurements to determine whether the polishingprocess yielded the desired topography, thickness, and uniformity.However, these pre- and post-polishing measurements are labor intensiveand result in a low product throughput.

It is also important to be able to determine when an end point has beenreached, such that polishing can stop. Optical scanning of a wafer and apattern recognition system could be employed. Using prior art patternrecognition techniques, the wafer would be moved so that multiple pointsof the wafer could be scanned. A problem with moving the wafer is thatslurry on the wafer would dry while the wafer is being scanned, and ifthe wafer is out of specification, it would be difficult to furtherpolish the wafer after the slurry has dried on the wafer.

The present invention provides a polishing system and method whichsignificantly reduces the problems associated with non-uniform removaland monitoring of the polishing process, and provides variousend-pointing techniques for determining whether polishing of a wafer iscomplete.

SUMMARY OF THE INVENTION

The invention provides a system for polishing a semiconductor wafer. Inaccordance with one aspect of the invention, the system comprises anoptical measurement system including a liquid filled, wafer receivingarea which selectively receives the wafer.

In accordance with one aspect of this invention, a system for polishinga semiconductor wafer comprises a wafer polishing assembly for polishinga face of a semiconductor wafer at a polishing rate and a polishinguniformity. The wafer polishing assembly has a plurality of controllableoperational parameters that upon variation change the polishing rate andpolishing uniformity. The system also comprises a controller operablycoupled to the wafer polishing assembly for monitoring and managing insitu at least one of the operational parameters of the wafer polishingassembly. A processor is operably coupled to the controller fordetermining a set of desired operational parameters based on themonitored operational parameters and for outputting control informationindicative of the desired operational parameters to the controller. Thecontroller adjusts in situ at least one of the operational parameters ofthe wafer polishing assembly in response to the control information fromthe processor to effectuate a new polishing rate and a new polishinguniformity as the wafer polishing assembly continues to polish the faceof the semiconductor wafer.

These operational parameters include platen rotational velocity, waferrotational velocity, the polishing path of the wafer, the wafer speedacross the platen, the down force exerted on the wafer, slurrycomposition, slurry flow rate, and temperature at the wafer surface.

According to another aspect of this invention, a system for polishing asemiconductor wafer comprises a rotatable platen subassembly whichdefines a polishing area and a drive mechanism coupled to rotate theplaten subassembly at a platen velocity. The system further comprises apolishing head for supporting a semiconductor wafer and holding a faceof the semiconductor wafer in contact with the platen subassembly topolish the wafer face whereby individual regions of the wafer face havedifferent polishing rates. The polishing head has pressure applicatorsfor applying various localized pressures on individual regions of thesemiconductor wafer to cause the semiconductor wafer to conform thewafer face to a selected contour. The system also comprises a polishcontrol subsystem for monitoring in situ the polishing rates at variousregions of the semiconductor wafer and adjusting in situ at least one ofthe platen velocity and the individual localized pressures applied tothe semiconductor wafer to change the polishing rates of the individualregions of the semiconductor wafer.

In another aspect of the invention, a system for polishing asemiconductor wafer comprises a wafer polishing assembly for polishing aface of a semiconductor wafer at a polishing rate and a polishinguniformity, the wafer polishing assembly including a platen subassemblydefining a polishing area, a slurry supply system delivering a slurry tothe polishing area, and a polishing head selectively supporting asemiconductor wafer and holding a face of the semiconductor wafer incontact with the platen subassembly; and an optical measurement systemincluding a liquid filled, wafer receiving area which selectivelyreceives the wafer, the optical measurement system measuring filmthickness at multiple different locations on the wafer face while thewafer is under a liquid, wherein drying of the wafer is impeded whilethe film thickness measurements are taken.

In another aspect of the invention, a system for polishing asemiconductor wafer comprises a wafer polishing assembly for polishing aface of a semiconductor wafer at a polishing rate and a polishinguniformity, the wafer polishing assembly including a platen, a polishinghead which supports the semiconductor wafer, and a polishing headdisplacement mechanism which moves the polishing head and wafer acrossthe platen, the wafer polishing assembly having a plurality ofcontrollable operational parameters that upon variation change thepolishing rate and polishing uniformity; a controller operably coupledto the wafer polishing assembly for monitoring and managing in situ atleast one of the operational parameters of the wafer polishing assembly;a processor operably coupled to the controller for determining a set ofdesired operational parameters based on the monitored operationalparameters and outputting control information indicative of the desiredoperational parameters to the controller, the controller adjusting insitu at least one of the operational parameters of the wafer polishingassembly in response to the control information from the processor toeffectuate a new polishing rate and a new polishing uniformity as thewafer polishing assembly continues to polish the face of thesemiconductor wafer; and an optical measurement system selectivelymeasuring film thickness at multiple different locations on the waferface, the optical measurement system including a liquid filled waferreceiving area keeping the wafer face wet while the film thicknessmeasurements are taken.

In another aspect of the invention, a system for polishing asemiconductor wafer comprises a wafer polishing assembly for polishing aface of a semiconductor wafer at a polishing rate and a polishinguniformity, the wafer polishing assembly including a platen, a polishinghead which supports the semiconductor wafer, and a polishing headdisplacement mechanism which moves the polishing head and wafer acrossthe platen, the wafer polishing assembly having a plurality ofcontrollable operational parameters that upon variation change thepolishing rate and polishing uniformity; a controller operably coupledto the wafer polishing assembly for monitoring and managing in situ atleast one of the operational parameters of the wafer polishing assembly;a processor operably coupled to the controller for determining a set ofdesired operational parameters based on the monitored operationalparameters and outputting control information indicative of the desiredoperational parameters to the controller, the controller adjusting insitu at least one of the operational parameters of the wafer polishingassembly in response to the control information from the processor toeffectuate a new polishing rate and a new polishing uniformity as thewafer polishing assembly continues to polish the face of thesemiconductor wafer; and a detector operating on the wafer andcommunicating with the processor to determine whether polishing of thewafer is complete, the detector including an optical measurement systemmeasuring film thickness at multiple different locations on the waferface while the wafer is under a liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more preferred forms of the invention are described herein withreference to the accompanying drawings. Like components and features arereferenced by like numerals throughout the drawings. The drawings arebriefly described below.

FIG. 1 is a diagrammatic perspective view of a polishing systemaccording to the invention.

FIG. 2 is a diagrammatic side view of the polishing system.

FIG. 3 is a diagrammatic side view of a polishing head according toanother aspect of this invention. The polishing head has multiplepressure applicators, and FIG. 3 shows the pressure applicators in theirretracted positions.

FIG. 4 is a diagrammatic side view similar to FIG. 3 and illustratessome of the pressure applicators in extended positions.

FIG. 5 is an enlarged diagrammatic side view of a pressure applicatorfor use in the FIG. 3 polishing head according to one embodiment of thisinvention.

FIG. 6 is an enlarged diagrammatic side view of a pressure applicatorfor use in the FIG. 3 polishing head according to another embodiment/ofthis invention.

FIG. 7 is a diagrammatic perspective view of a polishing systemincluding an end point detector, according to another embodiment of thisinvention.

FIG. 8 is a diagrammatic side view of a polishing system including analternative rind point detector.

FIG. 9 is a diagrammatic side view of a polishing system including analternative end point detector.

FIG. 10 is a diagrammatic perspective view of a polishing systemincluding another alternative end point detector.

FIG. 11 is a diagrammatic top view of a polishing head and platensubassembly according to another aspect of this invention.

FIG. 12 is a diagrammatic sectional view of a polishing head taken alongline 12--12 of FIG. 11.

FIG. 13 is a top view showing a platen subassembly included in apolishing system according to another aspect of this invention.

FIG. 14 is a diagrammatic sectional view of the platen subassembly takenalong line 14--14 of FIG. 13.

FIG. 15 is a diagrammatic sectional view of a platen subassemblyaccording to another aspect of this invention.

FIG. 16 is a diagrammatic perspective view of a polishing systemaccording to another aspect of the invention.

FIG. 17 is a diagrammatic perspective view of a polishing systemaccording to another aspect of the invention.

FIG. 18 is a diagrammatic side view of a polishing system including analternative end point detector.

FIG. 19 is a diagrammatic perspective view of a polishing systemaccording to another aspect of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws "to promote the progressof science and useful arts" (Article 1, Section 8).

FIGS. 1-2 are diagrammatical illustrations of a polishing system 10 forpolishing a semiconductor wafer. In its preferred form, system 10includes a chemical or slurry supply system 50 for introducing achemical slurry into the polishing environment to facilitate waferpolishing. Accordingly, in its preferred form, system 10 is a chemicalmechanical planarization (CMP) apparatus. However, as will be moreapparent in the continuing discussion, this invention is also capable ofbeing practiced using mechanical polishing techniques withoutintroduction of a chemical slurry.

Polishing system 10 has a wafer polishing assembly 12 for polishing aface of a semiconductor wafer 14. Wafer polishing assembly 12 includes arotatable platen subassembly 16 that is rotated at a platen velocityV_(P) about a center axis 18 by a motor or other drive mechanism 20. Theplaten subassembly can be rotated in a clockwise direction x (FIG. 1) orin the counterclockwise direction. Platen subassembly 16 includes aplaten 22 and a pad 24 mounted on the platen. Both the platen 22 and pad24 are preferably circular. Pad 24 protects platen 22 from the chemicalslurry introduced during the polishing process, and is typically made ofblown polyurethane. As used in this disclosure, the term "platensubassembly" is intended to include both a platen without a pad (i.e.,for some mechanical planarization situations) and a platen provided witha pad (i.e., for chemical mechanical planarization situations).

Wafer polishing assembly 12 also includes polishing head subassembly 30which consists of polishing head 32 (FIG. 2), motor or other drivemechanism 34, and polishing head displacement mechanism 36. Polishinghead 32 supports semiconductor wafer 14 and holds the wafer face incontact with pad 24 of platen subassembly 16. Polishing head 32 appliesa controlled adjustable downward force F (as illustrated by arrow 38) topress semiconductor wafer 14 into pad 24 to facilitate polishing of thewafer face. Motor 34 rotates polishing head 32 and wafer 14 at a wafervelocity V_(W) in a clockwise rotational direction y which is preferablythe same rotational direction of platen subassembly 16 (although wafer14 can be rotated in the counterclockwise direction or opposite torotation of the platen subassembly as desired).

Polishing head displacement mechanism 36 moves polishing head 32 andwafer 14 under controlled force F across platen subassembly 16 asindicated by arrows 40 and 42. The wafer is moved at an adjustable rateand along a variable polishing path. The polishing path can be linear,sinusoidal, or a variety of other patterns. Polishing head displacementmechanism 36 is also capable of moving semiconductor wafer 14 along apolishing path to a location beyond the edge of pad 24 so that wafer 14"overhangs" the edge. This overhang arrangement permits wafer 14 to bemoved partially on and partially off pad 24 to compensate for polishingirregularities caused by relative velocity differential between thefaster moving outer portions and the slower moving inner portions ofplaten subassembly 16.

Polishing head 32 includes means for holding the semiconductor wafer 14.One example holding means is a vacuum-type mechanism which generates anegative vacuum force to draw the wafer against the polishing head. Thevacuum-type mechanism is helpful in initially lifting and positioningthe wafer on the polishing head. Once the wafer is positioned on thepolishing head and held in contact with the platen subassembly forpolishing, the vacuum force can be removed. The polishing head isdesigned with a friction surface, or alternatively includes a carrierpad, which engages the upper, non-exposed face of the wafer and thefriction force created between the polishing head and wafer effectivelyholds the wafer against the polishing head and causes the wafer torotate at the same velocity as the polishing head. Such polishing headsand carrier pads are of conventional design and are commerciallyavailable.

FIGS. 3-6 illustrate another polishing head 100 unique to this inventionwhich can be used in the polishing system 10. Polishing head 100 has awafer carrier 102 sized to accommodate semiconductor wafer 14. Wafercarrier 102 has a relatively flat surface and a surrounding, annularflange 104 which defines a holding area. An upper, backside, ornon-exposed face of semiconductor wafer 14 lies in juxtaposition withthe flat surface of the wafer carrier 102. A lower, frontside, orexposed face of wafer 14 is held in contact with pad 24 duringpolishing. Flange 104 is sized to extend partially along and aroundwafer 14 to assist in maintaining the wafer within the holding area.

Polishing head 100 also has one or more pressure applicators 106provided on the wafer carrier 102. The pressure applicators 106 areindividually controllable to move over a range of positions fromretracted positions (FIG. 3) to extended positions (FIG. 4, for some ofthe applicators). Under a preferred embodiment, a carrier pad is locatedover the wafer carrier 102 between the pressure applicators 106 and thewafer. The carrier pad induces an effective friction at the waferbackside to cause the wafer to rotate with the wafer carrier and notslip. The carrier pad is not shown for purposes of clarity in describingthe contour changing effect on the wafer caused by the individuallycontrollable pressure applicators.

The applicators 106 operatively engage the non-exposed face of thesemiconductor wafer (preferably, through the carrier pad) and, as movedtoward their extended positions, apply multiple isolated localizedpressures on individual regions of the wafer. The localized pressurescause the semiconductor wafer to bend or bow and alter the contour ofthe exposed face being held against pad 24.

Individual pressure applicators 106 preferably include a slidable pistonwhich controllably moves between a retracted and extended position.FIGS. 5 and 6 show two embodiments of a piston-based pressureapplicator. In FIG. 5, pressure applicator 120 comprises a solenoid orservomechanism 122 which operatively drives a piston 124 to a desiredposition in response to electrical signals received on input line(s)126. Piston 124 includes a shaft 128 and a flat, circular disk 130mounted to the shaft.

In FIG. 6, pressure applicator 140 comprises an "I"-shaped piston 142slidably mounted with a hollow, cylindrical housing 144. Piston 142 hasan upper disk 146 sized to fit closely within the interior surface ofhousing 144, a lower disk 148 positioned outside of housing 144, and ashaft 150 interconnecting the two disks. A spring 152 is disposed aboutshaft 150 between a bottom wall or floor of housing 144 and the upperdisk 146 to bias the piston 142 to its retracted position. Housing 144has an upper opening which is operatively coupled to a tube or conduit154 to provide fluid communication between the conduit 154 and thehousing chamber. A fluid (which can be gas or liquid) is transferredunder controlled pressure through conduit 154 against upper piston disk146, whereby the pressure is effective to overcome the bias of spring152 to cause the desired movement of piston 142.

As shown in FIGS. 3 and 4, applicators 106 are individually coupled toan applicator controller 108 via a suitable connecting means 110. Whenthe servomechanism pressure applicators 120 of FIG. 5 are used,applicator controller 108 consists of a servo-electric applicatorcontroller which generates electric signals that operatively positionthe servomechanism pressure applicators 120. The connecting means 110consists of a bus or conductors suitable to carry the electric signalsfrom the servo-electric applicator controller to individual applicatorsand to provide feedback. On the other hand, when pressure applicators140 of FIG. 6 are employed, applicator controller 108 consists of afluid force generator which outputs a fluid under a controlled pressure.The connecting means 110 consists of tubing or conduits to transferfluid under pressure from the fluid force generator to individualapplicators.

According to the polishing head of this invention, the polishing ratesof individual regions across the wafer face can be independentlycontrolled to effectuate the desired polishing results. Prior to thisinvention, the semiconductor experienced different polishing rates invarious regions across the wafer face caused by the polishingenvironment including such things as platen velocity, wafer velocity,slurry composition, type of material on the wafer face, the down forceapplied to the wafer, and wafer movement across the platen. Thisinvention is advantageous because it provides superior control inselectively isolating and changing the polishing rates of specificregions of the semiconductor wafer in a real-time manner duringpolishing while globally polishing the entire wafer.

With reference again to FIGS. 1 and 2, wafer polishing assembly 12 alsoincludes chemical supply system 50 for introducing a chemical slurry ofa desired composition. Chemical supply system 50 has a chemical storage52 for storing slurry and a conduit 54 for transferring the slurry fromchemical storage 52 to the polishing area atop platen subassembly 16.Chemical supply system 50 introduces slurry as indicated by arrow 56atop pad 24 at a selected flow rate. This chemical slurry provides anabrasive material which facilitates polishing of the wafer face, and ispreferably a composition formed of a solution including solid alumina orsilica. However, according to this invention, the composition can becontrollably altered to add or remove individual chemicals from theslurry, or to change the ratios within the composition.

Wafer polishing assembly 12 has a film thickness measurement device 60for measuring topography of the wafer face during polishing. Filmthickness measurement device 60 is preferably implemented in the form ofa laser interferometer measuring apparatus which employs interference oflight waves for purposes of measurement. The laser interferometermeasuring apparatus includes light transmitter/receiver units 62provided at the surface of the platen subassembly 16 which transmitlight at the wafer face and collect reflections therefrom. The laserapparatus also includes laser source and controller 64 which isoptically coupled to units 62. The laser apparatus is configured tomeasure thicknesses and contour of films and materials on the waferface. Apart from the laser apparatus, this invention also contemplatesother techniques and systems that can be used as a film thicknessmeasurement device including a system for measuring capacitance changeduring wafer polishing, a device for detecting friction change at thewafer surface, and an acoustic mechanism for measuring wave propagationas films and layers are removed during polishing.

Wafer polishing assembly 12 also includes a temperature sensor 90positioned to detect temperature within the polishing area atop the pad24.

Polishing system 10 further includes a polish control subsystem 70 formonitoring in situ the operating parameters of the polishing system andadjusting in situ one or more polishing parameters to effectuate thedesired polishing results for a particular semiconductor wafer. Theoperating parameters are such that variation of one or more of theparameters effectively changes the polishing rates and polishinguniformity across the wafer face.

Polish control subsystem 70 includes a system controller 72 and aprocessor 74. System controller 72 is operatively coupled to thecomponents of the system via connectors 76-82 (and various otherconnectors shown in FIGS. 7-16) to monitor and manage in real-time atleast one of the operational parameters. The parameters are input toprocessor 74 which determines the present state polishing status of thesemiconductor wafer, including polishing uniformity and variouspolishing rates across the wafer. Processor 74 then determines a set ofdesired operational parameters which effectuates the desired polishinguniformity and rates, and outputs control information indicative ofthese desired parameters. Processor 74 can be embodied as amicroprocessor, an ASIC, or some other processing means for determiningthe desired operational parameters. Processor 74 may includecomputational means for calculating specific parameters, memory look-uptables for generating values given the measured parameters, or neuralnetworks and fuzzy logic techniques for systematically arriving atoptimal parameters.

The controller 72 uses the control information to adjust the systemcomponents and thereby modify the operational parameters which will tendto subject the wafer to polishing conditions that more closelyapproximate the desired polishing uniformity and rates. Morespecifically, controller 72 is coupled to polishing head displacementmechanism 36 via connector 76 to monitor and controllably adjust in situthe polishing path of the semiconductor wafer and the speed at which thewafer is moved across the platen subassembly 16. Controller 72 iscoupled to motor 34 via connector 77 to monitor the motor rpm and wafervelocity imparted by the polishing head. Controller 72 commands themotor to speed up or slow down based on the information received fromprocessor 74. Controller 72 is coupled to motor 20 via connector 80 tomonitor the motor rpm and platen velocity of platen subassembly 16, andto adjust the speed of the platen subassembly as desired.

Controller 72 is connected to slurry supply means 50 via connector 79 tomonitor and adjust slurry composition and flow rate. Controller 72 iscoupled to temperature sensor 90 via connector 78 to receive feedbackinformation concerning temperature of the polishing environment andwafer surface. Connector 81 conveys control signals and feedbackinformation between controller 72 and film thickness measurement device60.

When system 10 is adapted to incorporate polishing head 100 of FIGS. 3and 4, applicator controller 108 is operatively coupled via connector 82to system controller 72. According to this embodiment, controller 72 canmake independent adjustments to one or more of the pressure applicators106 on head 100, causing manipulation of the wafer face contour. Thiscontrol permits regional or localized polishing with a semiconductorwafer.

Controller 72 works in conjunction with film thickness measurementdevice 60 to determine the polishing rates and uniformity across thewafer during real-time evaluations. This information is passed toprocessor 74 which then generates a map indicative of the polish ratesand/or uniformity across the semiconductor wafer face for use inadjusting system operational parameters. Preferably, this map isgenerated on a periodic basis. In one embodiment, such mapping isperformed using the techniques disclosed in U.S. Pat. No. 5,196,353,issued to Sandhu et al., assigned to the assignee of the presentinvention, and incorporated herein by reference. The technique disclosedin U.S. Pat. No. 5,196,353 involves using an infrared camera to detectinfrared waves emitted from a wafer and correlating this information tothe heat of various points on the wafer. Using this arrangement, therelative temperature at any point on the wafer is detected and mapped,and an infrared image of the surface of the wafer is developed.

In one embodiment, shown in FIG. 7, the system 10 further comprises anend point detector (or end point detection means) 160 operating on thewafer and communicating with the system controller 72, and thus with theprocessor 74, for determining if polishing of the wafer is complete. Inthe embodiment shown in FIG. 7, the end point detector comprises meansfor sensing a change in friction between the wafer and the polishingplaten. Such friction sensing is disclosed in detail in U.S. Pat. Nos.5,036,015, and 5,069,002 issued to Sandhu et al., assigned to theassignee of the present invention, and incorporated herein by reference.

More particularly, in the embodiment shown in FIG. 7, the end pointdetector 160 comprises a friction sensor 162 sensing friction betweenthe wafer and the polishing platen. The friction sensor 162 is incommunication with the controller 72 (and thereby with the processor 74)via conductor 164.

As the semiconductor wafer is rotated and pressed against the platensubassembly 16, the oxide surface of the wafer contacts the polishingpad 24 of the platen 22. The oxide surface of the wafer has a hardnessthat produces a coefficient of friction when contacting the pad 24,which depends in part on the amount and composition of the slurrydelivered by the slurry supply system 50. The coefficient of frictionremains substantially constant until the oxide is polished away to apoint where IC devices on the wafer are exposed. The IC devices may beof a harder material than the oxide surface of the wafer. A differentcoefficient of friction is thus present when the oxide is polished away.Similarly, the coefficient of friction is different for different filmsformed on the wafer. Such different coefficients of friction will bedetected using the friction sensor 162. More particularly, the change infriction is detected by the processor 74 which monitors friction overtime for given parameters (such as speed of the motor 34, speed of themotor 20, downforce F, etc.). By sensing the change in friction that isnot caused by a change in a controllable operating parameter, theprocessor 74 determines when an end point has been reached, andpolishing can stop. The desired end point is preprogrammed into theprocessor, and can be after the oxide surface is removed, or after acertain film formed on the substrate is removed.

In an alternative embodiment, shown in FIG. 8, the system includes anend point detector 200 comprising a current meter 202 electricallyconnected to the motor 34 and in communication with the controller 72(and thereby with processor 74) via conductor 204, or a current meter206 electrically connected to the motor 20 and in communication with thecontroller 72, or both current meters 202 and 206. The current meter 202and/or 204 indicates to the processor 74 a change in friction bydetecting a change in amperage through the motor 34 and/or 20.

In an alternative embodiment shown in FIG. 9, the system includes an endpoint detector (or end point detection means) 300 which comprises meansfor directing acoustic waves at the wafer, and means for receivingreflected acoustic waves from the wafer. The use of acoustic waves in anend point detector is disclosed in U.S. Pat. No. 5,240,552, issued to Yuet al., assigned to the assignee of the present invention, andincorporated herein by reference.

More particularly, in the illustrated embodiment, the means fordirecting acoustic waves at the wafer comprises an acoustic wavetransducer 302 connected to the controller 72 (and thus to the processor74) via line 306, and the means for receiving reflected acoustic wavescomprises an acoustic wave receiver 304 mounted to receive acousticwaves reflected from the wafer and connected to the controller 72 (andthus to the processor 74) via line 308. The transducer 302 converts anapplied electrical voltage into a mechanical strain producing anacoustical wave. In one embodiment, the transducer 302 comprises apiezoelectric transducer, such as a thin film transducer, that convertsa voltage into an acoustical wave. Similarly, in one embodiment, thereceiver 304 comprises a piezoelectric receiver, such as a thin filmreceiver, that converts a reflected acoustic wave into a voltage. In theillustrated embodiment, the acoustic waves are directed at the backsideof the wafer. In an alternative embodiment (not shown), the waves aredirected at the front of the wafer by causing the polishing headdisplacement mechanism 76 to move the wafer to a location where anacoustic transducer and receiver can act on the front of the wafer. Thisis, for example, off the platen or at predetermined location on theplaten where the transducer and receiver are located. The thickness ofthe wafer and the oxide layer on the wafer is determined by theprocessor 74 which analyzes the acoustic wave that is sent by thetransducer 302 and the acoustic wave that is received by the receiver304. More particularly, thickness is determined from the round trip timeinterval between the launch of an acoustical wave by the transducer 302and the reception of the reflected wave by the receiver 304, and thespeed of the acoustic waves through the layers of the wafer.

The amplitude as well as round trip time of the acoustic waves willchange after a film has been completely removed and a different filmlayer has been contacted. An end point that corresponds to theinterfaces of a different film of multiple layers of stacked films canbe detected, as well as the end point of an oxide layer. In oneembodiment, the planarization of a film is measured in real time bymeasuring a film thickness at several locations on the wafer.

The system controller 72 causes the transducer 302 to generateacoustical waves, and receives voltage signals from the receiver 304,the processor 74 communicates with the controller 72 to analyze theacoustical waves. More particularly, the controller 72 includes a pulsegenerator and amplifier driving the transducer 302, includes a low noiseamplifier amplifying the signal produced by the receiver 30, andincludes a lock in amplifier coordinating the signals generated by thepulse generator and received by the receiver 304.

In another alternative embodiment, shown in FIG. 9, the system 10comprises an end point detector (or end point detection means) 400comprising means for detecting temperatures of different areas of thewafer using an infrared camera during polishing to develop an infraredimage of the wafer. Such infrared mapping is disclosed in U.S. Pat. No.5,196,353, issued to Sandhu et al., assigned to the assignee of thepresent invention, and incorporated herein by reference.

More particularly, in the illustrated embodiment, the means fordetecting temperatures of different areas of the wafer comprises aninfrared camera 402 connected to the controller 72 (and thus to theprocessor 74) via line 404. The infrared camera 402 may be mounted tothe platen 22. In the illustrated embodiment, the operative portion(lens or window) of the infrared camera 402 is generally flush with, orslightly below, the top surface of the polishing pad 24 and faces thewafer. During polishing, the wafer is periodically moved by thepolishing head displacement mechanism 36 over the camera 402. The camera402 is of a type which contains an array or matrix of cells and iscapable of mapping temperatures at different locations, and hence atdifferent locations of the wafer.

In an alternative embodiment, a camera 406 (shown with dashed lines) isprovided instead of the camera 402, and is connected to the controller72 (and thus to the processor 74) via line 408. The camera 406 includesa lens or operative portion extending radially outwardly from the centerof the platen subassembly to the periphery of the platen subassembly. Inthis manner, the wafer is continuously scanned during polishing.

With either camera arrangement, infrared waves emitted from the waferare detected, and this information is correlated by the camera to theheat of various locations on the wafer. The infrared camera can eithercontinuously (e.g., video), or periodically (e.g., photograph) take animage of the wafer.

During polishing of a wafer, process heat is developed as a result offriction. The temperature of the wafer surface is largely dependent onfrictional force. Because different layers of the semiconductor materialare formed of different materials (e.g., metallic films, polysiliconfilms, insulators) which have different relative hardnesses, thecoefficient of friction and thus the temperature of the wafer willchange in response to contact with a different layer. For example,integrated circuit devices on the wafer are generally harder than theoxide coating on the integrated circuit devices. In addition to use indeveloping a thermal image of the wafer developed during the polishingprocess, such temperature differentials are used to detect planar endpoints on the wafer. More particularly, the camera is connected to thecontroller 72, and the processor 74 determines when a planar end pointhas been reached in response to an expected temperature differential.

For example, the coefficient of friction between the wafer and thepolishing pad 24, and thus the infrared image of the wafer, may begenerally constant until the oxide of the wafer is polished away to apoint where the surface of integrated circuits is exposed. At this time,the integrated circuits will contact the surface of the polishing pad.Because the integrated circuits are typically formed of harder materialthan the oxide coating, a different coefficient of friction occurs, andtemperature rises. This rise in temperature is detected by the infraredcamera and is used to control the operational parameters of the system10. Such a rise in temperature may occur at a particular location on thewafer where oxide is more thoroughly removed than other areas. Thepressure applicators can then be controlled to deform the wafer so thatoxide is also removed from the rest of the wafer. Other operationalparameters can also be adjusted based on the information provided by theinfrared camera.

In another alternative embodiment, shown in FIG. 11, the system 10further comprises means 500 for heating the wafer while the wafer faceis being polished. More particularly, the heating means 500 iscontrollably adjustable while the wafer is being polished, so that wafertemperature becomes another of the adjustable operational parameters.

In the embodiment shown in FIG. 11, the heating means 500 comprises aheater or means for heating the platen subassembly 16 while the wafer isbeing polished. More particularly, the heating means 500 comprises aheating filament 502 supported by the platen subassembly 16 in theillustrated embodiment. In the illustrated embodiment, the heatingfilament 502 is an element that evenly heats the platen, and that islocated in the platen just below the pad 24. Appropriate connectorspermit electrical connection to the filament 502, for energization ofthe filament, while allowing rotation of the filament 502 with theplaten. For example, circumferential connectors can be provided on theshaft that rotates the platen 22, and electrical contact can be madeusing brushes. The heating filament 502 is connected to the systemcontroller 72 (and therefore to the processor 74) via a connector 504.In alternative embodiments, controllably heated liquid (described belowin greater detail) or gas is introduced to the interior or exterior ofthe platen to controllably heat the platen.

In another alternative embodiment, shown in FIG. 12, the system 10further comprises a heater or means 600 for heating the wafer while thewafer face is being polished, in the form of means for heating thepolishing head 32 while the wafer is being polished. More particularly,the heating means 600 comprises a heating filament 602 supported by thepolishing head 100 in the illustrated embodiment. In the illustratedembodiment, the heating filament 602 is annular or arcuate. However,other alternative heating filament shapes can be employed. Appropriateconnectors permit electrical connection to the filament 602, forenergization of the filament, while allowing rotation of the filament602 with the polishing head 100. For example, circumferential connectorscan be provided on the shaft that rotates the polishing head 100, andelectrical contact can be made using brushes. In alternativeembodiments, controllably heated fluid (liquid or gas) is introduced tothe interior or exterior of the polishing head to heat the polishinghead. The heating filament 602 is connected to the system controller 72(and therefore to the processor 74) via a connector 604.

In another alternative embodiment, shown in FIGS. 13-14, the system 10further comprises a heater or means 700 for heating the wafer while thewafer face is being polished, in the form of a heater or means 704 forheating the platen subassembly 16 while the wafer is being polished.More particularly, platen subassembly 16 comprises a platen 22 having ahollow interior 702 (or a fluid passage in its interior), and the meansfor heating the platen subassembly 16 comprises means for flowing fluidthrough the hollow interior 702 and for controllably heating thetemperature of the fluid. In the illustrated embodiment, the fluid is aliquid; however, in alternative embodiments, the fluid is a gas. In theembodiment shown in FIG. 14, the means for flowing fluid and forcontrollably heating the temperature of the fluid comprises the hollowinterior or fluid passage 702, a pump 706 in fluid communication withthe hollow interior or fluid passage 702 and conducting fluid throughthe hollow interior, and a heating element 708 heating the fluid. Thepump 706 and the heating element 708 are connected to the controller 72,and thus to the processor 74.

In the illustrated embodiment, the fluid is a liquid, and the system 10further includes, as appropriate, a sump or collection area 710 whichmay be generally annular, a conduit 711 directing collected liquid tothe pump 706, nozzles 712 directed at the hollow interior or fluidpassage 702; and conduits 714 from the pump 706 to the nozzles 712.Bearings may be provided where appropriate. The heating element 708 canheat fluid either before or after it passes through the pump 706. In anembodiment where the fluid is a gas, the heating element 708, nozzles712, sump 710, and conduit 711 can be replaced with a blower unit whichselectively forces heated air at a controllable rate and/or controllabletemperature toward the hollow interior or fluid passage 702.

The platen subassembly 16 includes any appropriate mounting structure716 with which the platen subassembly 16 is supported for rotation witha spindle 718 driven by the motor 20. For example, the platensubassembly 16 can be threaded, friction fit, welded, or otherwisesecured to the spindle 718 or other support structure.

In alternative embodiments (not shown), the platen subassembly 16 issupported such that the hollow interior 702 is in direct contact with apool of liquid, which pool of liquid is selectively heated or cooled.

In another alternative embodiment, shown in FIG. 15, the system 10comprises a heater or means 800 for heating the wafer while the waferface is being polished, in the form of a heater or means 802 for heatingthe platen subassembly 16 while the wafer is being polished. In theembodiment shown in FIG. 15, the platen subassembly 16 comprises aplaten 22 having a fluid passage 804 in its interior, and the means forheating the platen subassembly 16 comprises means for flowing fluidthrough the fluid passage 804 and for controllably heating thetemperature of the fluid. In the illustrated embodiment, the platensubassembly 16 is supported for rotation by a rotatable spindle 818having a hollow interior 820, and fluid is introduced into the fluidpassage 804 via the hollow interior 820 of the spindle 818. Optionally,a tube 822 is provided in the hollow interior 820, and the spindle 818rotates about the tube 822. In the illustrated embodiment, the fluid isa liquid; however, in alternative embodiments, the fluid is a gas. Inthe embodiment shown in FIG. 15, the means for flowing fluid and forcontrollably heating the temperature of the fluid comprises the fluidpassage 804, the tube 822 or hollow 820 in fluid communication with thefluid passage 804, a pump 806 in fluid communication with the fluidpassage 804 or hollow 820, and a heating element 808 heating the fluid.The pump 806 and the heating element 808 are connected to the controller72, and thus to the processor 74.

In the illustrated embodiment, the fluid is a liquid, and the system 10further includes, as appropriate, a sump or collection area 810, aconduit 812 directing collected liquid to the pump 806, and bearings, asappropriate. The heating element 820 can heat fluid either before orafter it passes through the pump 806. In an embodiment where the fluidis a gas, the heating element 808, sump 810, and conduits 812 can bereplaced with a blower unit in fluid communication with the hollow 820or tube 822.

In another alternative embodiment, shown in FIG. 16, the system 10comprises a heater or means 900 for heating the wafer while the waferface is being polished, in the form of means for changing thecomposition of the slurry delivered by the slurry supply system 50. Moreparticularly, in the embodiment shown in FIG. 16, the slurry supplysystem 50 comprises multiple chemical storage areas 902, 904, 906, etc.,which contain slurries of different compositions, and a controllablestorage selector 908 which supplies a slurry of a selected compositionto the conduit 54. The storage selector 908 is connected to the systemcontroller 72, and thus to the processor 74, via a line 910. Because thedifferent slurries contained in the chemical storage areas 902, 904,906, etc. have different compositions, changing the chemical slurryresults in a change in friction between the wafer and the polishing pad,and therefore in a change in temperature of the wafer while the wafer isbeing polished.

In another alternative embodiment, shown in FIG. 17, the system 10comprises a heater or means 1000 for heating the wafer while the waferface is being polished, in the form of means for heating the slurrybefore it is supplied to the wafer. More particularly, in the embodimentshown in FIG. 17, the system 10 comprises a heater 1002 which heats theslurry from the chemical storage 52 before it is supplied to the wafer,to heat the wafer. The heater 1002 is connected to the system controller72, and thus to the processor 74, via a line 1004.

In yet another alternative embodiment (FIG. 1), the system 10 comprisesheating means for controllably heating the wafer as it is being polishedin the form of means for adjusting the force between the polishing headand the platen subassembly. More particularly, if the processor 74determines that the temperature of the wafer should be changed, itinstructs the system controller 72 to act on the polishing headdisplacement mechanism 36 to adjust the force between the wafer and thepolishing pad 24. The change in force results in a change of frictionbetween the wafer and the polishing pad, which in turn results in achange in temperature of the wafer while the wafer is being polished.

In one embodiment, shown in FIG. 18, the system 10 further comprises anend point detector (or end point detection means) 1100 operating on thewafer and communicating with the system controller 72, and thus with theprocessor 74, for determining if polishing of the wafer is complete. Inthe embodiment shown in FIG. 18, the end point detector comprises anoptical measurement system 1102 including a liquid filled, waferreceiving area 1104 which selectively receives the wafer. Such anoptical measurement system 1102 is sold by Nova Inc., under the productname NovaScan 210-CMP Monitoring System.

The system 10 shown in FIG. 18 further includes a robot 1106, which isconnected to the system controller 72, and thus to the processor 74, vialine 1108. The robot 1106 periodically moves the wafer from the platento the wafer receiving area 1104 to determine whether polishing iscomplete, or to otherwise evaluate the wafer. Robots for moving wafersare commonly known in the wafer polishing art, and the robot 1106 willnot be discussed herein in great detail. The robot 1106 is capable oflifting, translating, and depositing a wafer, and can be of a type thatuses suction to engage the rear surface of a wafer, or of a type thatengages an edge of the wafer.

The optical measurement system 1102 measures film thickness at multipledifferent locations on the wafer face while the wafer face is in contactwith a liquid 1110 in the wafer receiving area 1104, which can be water,a slurry of the type provided by the slurry delivery system 50, oranother appropriate liquid for keeping the wafer face wet. Thus, dryingof the wafer is avoided while the film thickness measurements are taken.If the wafer requires further polishing, it can be returned to theplaten by the robot 1106 while still wet. If no provision was made tokeep the wafer wet, the slurry would dry on the wafer face makingfurther polishing difficult or impossible.

It is not practical to move a wet wafer and still take opticalmeasurements. Thus, in the illustrated embodiment, the opticalmeasurement system 1102 comprises movable optics 1112 which scan thewafer face while the wafer face remains stationary. More particularly,the optical measurement system 1102 includes positioning circuitry 1122,under the control of optical measurement system computer 1124, whichmoves the optics 1112 along x and y axes which are parallel to the faceof the wafer (e.g., in the illustrated embodiment, the optics 1112 movesalong horizontal axes). The optics 1112 include a lens 1114, and an autofocus system 1116 which focuses the lens 1114. A photosensor array 1118receives information from the wafer via the lens 1114, and a lightsource 1120 provides illumination.

The optical measurement system 1102 comprises an optical window 1126,such as a transparent (e.g., glass) window, at the bottom of the waferreceiving area 1104. Thus, the wafer receiving area 1104 is located onone side of the window, and the optics are located on the other side ofthe window 1126.

The optical measurement system 1102 includes a pattern recognitionsystem (circuitry) 1128 receiving information from the photosensor array1118 via measurement circuitry 1130. The measurement circuitry 1130charges and discharges cells in the photosensor array 1118, and takesmeasurements in a known manner. The pattern recognition circuitry 1128compares the thickness of a film on the wafer at various locations ofthe wafer with desired thicknesses (process specifications) that arepreprogrammed into the processor 74. Such pattern recognition circuitryis known in the art, and included in the NovaScan 210 CMP MonitoringSystem. Other companies that manufacture appropriate pattern recognitioncircuitry include Tencor, Metrologix, and Thermawave. If polishing iscomplete, the robot 1106 transports the wafer to an area which containscompleted wafers, such as an unload cassette. If the wafer isunderpolished, the robot 1106 transports the wafer back to the platenfor additional polishing. If the wafer is either overpolished orunderpolished, the system 10 modifies the sequence and duration ofoperational parameters (e.g., the polishing recipe) for the next waferthat is to be polished so as to improve the polishing results. When thenext wafer is polished and measured by the optical measurement system1102, it should be closer to being appropriately (completely) polished.

The optical measurement system 1102 takes measurements at multiplepoints on a wafer at a very fast rate, so that the throughput of thesystem 10 is not adversely affected.

As the semiconductor wafer is rotated and pressed against the platensubassembly 16, the oxide surface of the wafer contacts the polishingpad 24 of the platen 22. The oxide surface of the wafer has a thicknessthat changes as the wafer is polished. The oxide thickness changes asthe oxide is polished away. Similarly, thicknesses measurements can betaken, using the optical measurement system 1102, for different filmsformed on the wafer. Such different thicknesses can be detected by theoptical measurement system 1102 to terminate polishing when an end pointhas been reached. The desired end point is preprogrammed into theprocessor 74, and can be after the oxide surface is removed, or after acertain film formed on the substrate is removed.

In the embodiment shown in FIG. 18, the optical window is spaced apartfrom the platen, and the robot selectively moves the wafer between theplaten and the wafer receiving area. In an alternative embodiment, shownin FIG. 19, the optical window is located on the platen, the platenincludes a polishing surface 1200 spaced apart from the optical window1126, and the polishing head selectively moves the wafer between thepolishing surface 1200 and the optical window 1126.

More particularly, in the embodiment shown in FIG. 19, the opticalwindow 1126 is located at the center of the platen.

Various combinations of the components of the alternative embodiments isalso contemplated, and is within the scope of the invention.

System 10 is therefore a fully automatic, computer driven apparatus thatcan polish a wafer, monitor results in situ, and make appropriatemodifications in a real-time manner without any human intervention. Theinvention is advantageous over prior art polishing apparatus because itlargely reduces the number of pre- and post-polishing measurements andsignificantly enhances throughput. The system enhances both efficiencyand quality.

In compliance with the statute, the invention has been described inlanguage necessarily limited in its ability to properly convey theconceptual nature of the invention. Because of this inherent limitationof language, it must be understood that the invention is not necessarilylimited to the specific features shown and described, since the meansand methods herein disclosed comprise merely preferred forms of puttingthe invention into effect. The invention is, therefore, claimed in anyof its forms or modifications within the proper scope of the appendedclaims appropriately interpreted in accordance with the doctrine ofequivalents.

We claim:
 1. A system for polishing a semiconductor wafer, the systemcomprising:a wafer polishing assembly for polishing a face of asemiconductor wafer at a polishing rate and a polishing uniformity, thewafer polishing assembly including a platen subassembly defining apolishing area, a slurry supply system delivering a slurry to thepolishing area, and a polishing head selectively supporting asemiconductor wafer and holding a face of the semiconductor wafer incontact with the platen subassembly; an endpoint detector operating onthe wafer to determine whether polishing of the wafer is complete, theendpoint detector having an optical measurement system including anoptical window spaced apart from the platen subassembly, including aliquid filled, wafer receiving area, on one side of the optical window,which selectively receives the wafer, and including optics on the otherside of the window, and movable relative to the wafer receiving area,which scan the wafer face while the wafer face remains stationary, theoptical measurement system measuring film thickness at multipledifferent locations on the wafer face while the wafer face is in contactwith a liquid, wherein drying of the wafer is impeded while the filmthickness measurements are taken; and a robot selectively moving thewafer between the platen subassembly and the wafer receiving area.
 2. Asystem for polishing a semiconductor wafer comprising:a wafer polishingassembly for polishing a face of a semiconductor wafer at a polishingrate and a polishing uniformity, the wafer polishing assembly includinga platen, a polishing head which supports the semiconductor wafer, and apolishing head displacement mechanism which moves the polishing head andwafer across the platen along an adjustable polishing path, the waferpolishing assembly having a plurality of controllable operationalparameters, including polishing path, that upon variation change thepolishing rate and polishing uniformity; a controller operably coupledto the wafer polishing assembly for monitoring and managing in situ atleast one of the operational parameters of the wafer polishing assembly;a processor operably coupled to the controller for determining a set ofdesired operational parameters based on the monitored operationalparameters and outputting control information indicative of the desiredoperational parameters to the controller, the controller adjusting insitu at least one of the operational parameters of the wafer polishingassembly in response to the control information from the processor toeffectuate a new polishing rate and a new polishing uniformity as thewafer polishing assembly continues to polish the face of thesemiconductor wafer; a detector operating on the wafer and communicatingwith the processor to determine whether polishing of the wafer iscomplete, the detector including an optical measurement system includinga wafer receiving area spaced apart from the platen and measuring filmthickness at multiple different locations on the wafer face while thewafer is under a liquid, the optical measurement system includingmovable optics which scan the wafer face while the wafer face remainsstationary and; a robot selectively moving the wafer between the platenand the wafer receiving area.
 3. A system in accordance with claim 2wherein the optical measurement system includes a pattern recognitionsystem.
 4. A system in accordance with claim 2 wherein the opticalmeasurement system comprises an optical window, and wherein the waferreceiving area is liquid filled and is located above the window, andwherein the optics are located below the window.